It is well known that phyllosilicates, such as smectite clays, e.g., sodium montmorillonite and calcium montmorillonite, can be treated with organic molecules, such as organic ammonium ions, to intercalate the organic molecules between adjacent, planar silicate layers, for bonding the organic molecules with a polymer, for intercalation of the polymer between the layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The thus-treated, intercalated phyllosilicates, having interlayer spacings of at least about 10-20 .ANG. and up to about 100 Angstroms, then can be exfoliated, e.g., the silicate layers are separated, e.g., mechanically, by high shear mixing. The individual silicate layers, when admixed with a matrix polymer, before, after or during the polymerization of the matrix polymer, e.g., a polyamide--see U.S. Pat. Nos. 4,739,007; 4,810,734; and 5,385,776--have been found to substantially improve one or more properties of the polymer, such as mechanical strength and/or high temperature characteristics.
Exemplary prior art composites, also called "nanocomposites", are disclosed in published PCT disclosure of Allied Signal, Inc. WO 93/04118 and U.S. Pat. No. 5,385,776, disclosing the admixture of individual platelet particles derived from intercalated layered silicate materials, with a polymer to form a polymer matrix having one or more properties of the matrix polymer improved by the addition of the exfoliated intercalate. As disclosed in WO 93/04118, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer. Such quaternary ammonium cations are well known to convert a highly hydrophilic clay, such as sodium or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules. A publication that discloses direct intercalation (without solvent) of polystyrene and poly(ethylene oxide) in organically modified silicates is Synthesis and Properties of Two-Dimensional Nanostructures by Direct Intercalation of Polymer Melts in Layered Silicates, Richard A. Vaia, et al., Chem. Mater., 5:1694-1696(1993). Also as disclosed in Adv. Materials, 7, No. 2: (1985), pp, 154-156, New Polymer Electrolyte Nanocomposites: Melt Intercalation of Poly(Ethylene Oxide) in Mica-Type Silicates, Richard A. Vaia, et al., poly(ethylene oxide) can be intercalated directly into Na-montmorillonite and Li-montmorillonite by heating to 80.degree. C. for 2-6 hours to achieve a d-spacing of 17.7 .ANG.. The intercalation is accompanied by displacing water molecules, disposed between the clay platelets, with polymer molecules. Apparently, however, the intercalated material could not be exfoliated and was tested in pellet form. It was quite surprising to one of the authors of these articles that exfoliated material could be manufactured in accordance with the present invention.
Previous attempts have been made to intercalate polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and poly(ethylene oxide) (PEO) between montmorillonite clay platelets with little success. As described in Levy, et al., Interlayer Adsorption of Polyvinylpyrrolidone on Montinorillonite, Journal of Colloid and Interface Science, Vol. 50, No. 3, March 1975, pages 442-450, attempts were made to sorb PVP (40,000 average M.W.) between monojonic montmorillonite clay platelets (Na, K, Ca and Mg) by successive washes with absolute ethanol, and then attempting to sorb the PVP by contact with 1% PVP/ethanol/water solutions, with varying amounts of water, via replacing the ethanol solvent molecules that were sorbed in washing (to expand the platelets to about 17.7 .ANG.). Only the sodium montmorillonite had expanded beyond a 20 .ANG. basal spacing (e.g., 26 .ANG. and 32 .ANG.), at 5.sup.+ % H.sub.2 O, after contact with the PVP/ethanol/H.sub.2 O solution. It was concluded that the ethanol was needed to initially increase the basal spacing for later sorption of PVP, and that water did not directly affect the sorption of PVP between the clay platelets (Table II, page 445), except for sodium montmorillonite. The sorption was time consuming and difficult and met with little success.
Further, as described in Greenland, Adsorption of Polyvinyl Alcohols by Montmorillonite, Journal of Colloid Sciences, Vol. 18, pages 647-664 (1963), polyvinyl alcohols containing 12% residual acetyl groups could increase the basal spacing by only about 10 .ANG. due to the sorbed polyvinyl alcohol (PVA). As the concentration of polymer in the intercalant polymer-containing solution was increased from 0.25% to 4%, the amount of polymer sorbed was substantially reduced, indicating that sorption might only be effective at polymer concentrations in the intercalant polymer-containing composition on the order of 1% by weight polymer, or less. Such a dilute process for intercalation of polymer into layered materials would be exceptionally costly in drying the intercalated layered materials for separation of intercalate from the polymer carrier, e.g., water, and, therefore, apparently no further work was accomplished toward commercialization.
In accordance with one embodiment of the present invention, intercalates are prepared by contacting a phyllosilicate with a monomeric organic compound having a long chain alkyl radical (C.sub.10 + alkyl) and having an electrostatic functionality on one end of the molecule that provides a dipole moment that is greater than the dipole moment of water. Exemplary of such electrostatic functionalities include a hydroxyl; a polyhydroxyl; a carbonyl such as carboxylic acids, and salts thereof; polycarboxylic acids and salts thereof; aldehydes; ketones; amines; amides; ethers; esters; lactams; lactones; anhydrides; nitrites; n-alkyl halides; pyridines; and mixtures thereof.
In accordance with an important feature of the present invention, best results are achieved by mixing the layered material with a polar monomeric organic intercalant compound, having a C.sub.10 + alkyl group and a dipole moment on one end of the molecule greater than the dipole moment of water, or greater than 1.85 debyes (&gt;1.85 D), in a concentration of at least about 2%, preferably at least about 5% by weight polar, long chain alkyl monomeric organic intercalant compound, more preferably at least about 10% by weight polar, long chain alkyl monomeric organic intercalant compound, and most preferably about 30% to about 80% by weight, based on the weight of polar, long chain alkyl monomeric organic intercalant compound and carrier (e.g., water, with or without an organic solvent for the polar, long chain alkyl monomeric compound) to achieve better sorption of the monomeric organic intercalant compound between the platelets of the layered material. Dipole moments can be found, or calculated, in accordance with the publication "Selected Values of Electric Dipole Moments for Molecules in the Gas Phase", Nelson, Lide and Maryott, National Reference Data Series--National Bureau of Standards (NSRDS-NBS 10), hereby incorporated by reference. The publication is available from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402. One debye unit (D)=10.sup.-18 electrostatic units of charge.times.centimeters. The conversion factor to the Systeme International is 1D=3.33564 .times.10.sup.-24 coulombmeter. Regardless of the concentration of monomeric organic intercalant compound in aqueous liquid, the intercalating composition should have a polar, long chain monomeric organic intercalant compound:layered material weight ratio of at least 1:20, preferably at least 1:10, more preferably at least 1:5, and most preferably about 1:4 to achieve electrostatic complexing of the electrostatic monomeric organic intercalant compound with an inner surface of a platelet of the layered material to achieve efficient intercalation of the monomeric organic intercalant compound between adjacent platelets of the layered material. The polar, long chain (C.sub.10 + alkyl) monomeric organic intercalant compound sorbed between and bonded to (complexed with) the silicate platelets causes surprising separation or added spacing between adjacent silicate platelets.
For simplicity of description, the above-described C.sub.10 + alkyl monomeric organic intercalant compounds having at least one molecule end that has a dipole moment greater than the dipole moment of water are hereinafter called the "intercalant" or "intercalant monomer" or "monomer intercalant". The monomer intercalant will be sorbed sufficiently to increase the interlayer spacing of the phyllosilicate in the range of about 5 .ANG. to about 100 .ANG., preferably at least about 10 .ANG. for easier and more complete exfoliation, in a commercially viable process, regardless of the particular layered material, e.g., phyllosilicate, or intercalant monomer.
In accordance with the present invention, it has been found that a phyllosilicate, such as a smectite clay, can be intercalated sufficiently for subsequent exfoliation by sorption of organic monomer compounds that have an alkyl group of at least 10 carbon atoms, and include one end of the molecule having a dipole moment greater than the dipole moment of water to provide bonding between a polar end of one or two intercalant monomer molecules and the Na.sup.+ cations of the inner surfaces of the platelets of the layered material, e.g., phyllosilicate. Sorption and metal cation attraction or bonding between two polar end groups of the intercalant monomer molecules and the interlayer Na.sup.+ cations of the phyllosilicate; or the bonding between the interlayer Na.sup.+ cations in hexagonal or pseudohexagonal rings of the smectite platelet layers and an intercalant monomer aromatic ring structure, is provided by a mechanism selected from the group consisting of ionic complexing; electrostatic complexing; chelation; hydrogen bonding; ion-dipole; dipole/dipole; Van Der Waals forces; and any combination thereof.
Such bonding, via one or more metal (Na.sup.+) cations of the phyllosilicate sharing electrons with one or two electronegative atoms of one or two polar ends of C.sub.10 + alkyl monomer intercalant molecules, on an inner surface of one or both adjacent phyllosilicate platelets surprisingly provides rigid intercalant monomer molecules extending perpendicularly from the phyllosilicate platelet surfaces, and increases the interlayer spacing between adjacent silicate platelets or other layered material to at least about 5 .ANG., preferably to at least about 10 .ANG., more preferably to at least about 20 .ANG., and most preferably in the range of about 30 .ANG. to about 45 .ANG., while consuming surprisingly little monomer intercalant in relation to the increased basal spacing achieved. The electronegative atoms at the polar end of the intercalant monomer molecules that coordinate to surround the platelet Na.sup.+ ions can be, for example, oxygen, sulfur, nitrogen, halogen, and combinations thereof.
Such intercalated phyllosilicates easily can be exfoliated into individual phyllosilicate platelets before or during admixture with a liquid carrier or solvent, for example, one or more monohydric alcohols, such as methanol, ethanol, propanol, and/or butanol; polyhydric alcohols, such as glycerols and glycols, e.g., ethylene glycol, propylene glycol, butylene glycol, glycerine and mixtures thereof; aldehydes; ketones; carboxylic acids; amines; amides; and other organic solvents, for delivery of the solvent in a thixotropic composition, or for delivery of any active hydrophobic or hydrophilic organic compound, such as a topically active pharmaceutical, dissolved or dispersed in the carrier or solvent, in a thixotropic composition; or the intercalates and/or exfoliates thereof can be admixed with a polymer or other organic monomer compound(s) or composition to increase the viscosity of the organic compound or provide a polymer/intercalate and/or polymer/exfoliate composition to enhance one or more properties of a matrix polymer.